Russian investigators have filed criminal charges against an assistant of opposition leader Sergei Udaltsov, as part of an ongoing crackdown against opponents of President Vladimir Putin.

Russia's Investigations Committee said Thursday it charged Left Front party member Konstantin Lebedev with plotting mass riots to overthrow the government. Authorities detained Lebedev and party leader Udaltsov on Wednesday and searched their homes in response to allegations that both men were involved in such a plot. Udaltsov was released a short time later and ordered to stay in Moscow, but Lebedev remained in custody.

Russian investigators said Lebedev refused to acknowledge guilt or provide any evidence. Udaltsov also denied wrongdoing and called the allegations a "monstrous outrage."

Russian authorities said Wednesday they opened the investigation because of a documentary broadcast last week on a Putin-allied television channel. The program included purported hidden camera footage of Udaltsov meeting with Georgian officials to raise money for anti-Putin protests in Russia.

Udaltsov has said he met with many people recently as part of what he called legal fundraising activities. He also has dismissed the footage in the documentary as doctored. But the Investigations Committee has said it studied the footage and concluded it was genuine.